syntax: ruby: colorize embedded documentation as a comment

Vim and Emacs do this too.

Reference:
  https://en.wikibooks.org/wiki/Ruby_Programming/Syntax/Lexicology

Also, when colorizing double-hashed comments specially anyway,
avoid colorizing ##.* as a single-hashed comment.

syntax/ruby.nanorc

@@ -30,8 +30,9 @@ color brightgreen "#\{[^}]*\}"
 color green "'([^']|\\')*'" "%[qw](\{[^}]*\}|\([^)]*\)|<[^>]*>|\[[^]]*\]|\$[^$]*\$|\^[^^]*\^|![^!]*!)"
 
 # Comments.
-color cyan "#([^{].*|$)"
+color cyan "#([^{#].*|$)"
 color brightcyan "##([^{].*|$)"
+color cyan start="^=begin\>" end="^=end\>"
 
 # Some common markers.
 color brightcyan "XXX|TODO|FIXME|\?\?\?"